P. L. Pritchett

Bio: P. L. Pritchett is an academic researcher from University of California, Los Angeles. The author has contributed to research in topics: Magnetic reconnection & Current sheet. The author has an hindex of 33, co-authored 73 publications receiving 4253 citations.

Geospace Environmental Modeling (GEM) magnetic reconnection challenge

Abstract: The Geospace Environmental Modeling (GEM) Reconnection Challenge project is presented and the important results, which are presented in a series of companion papers, are summarized. Magnetic reconnection is studied in a simple Harris sheet configuration with a specified set of initial conditions, including a finite amplitude, magnetic island perturbation to trigger the dynamics. The evolution of the system is explored with a broad variety of codes, ranging from fully electromagnetic particle in cell (PIC) codes to conventional resistive magnetohydrodynamic (MHD) codes, and the results are compared. The goal is to identify the essential physics which is required to model collisionless magnetic reconnection. All models that include the Hall effect in the generalized Ohm's law produce essentially indistinguishable rates of reconnection, corresponding to nearly Alfvenic inflow velocities. Thus the rate of reconnection is insensitive to the specific mechanism which breaks the frozen-in condition, whether resistivity, electron inertia, or electron thermal motion. The reconnection rate in the conventional resistive MHD model, in contrast, is dramatically smaller unless a large localized or current dependent resistivity is used. The Hall term brings the dynamics of whistler waves into the system. The quadratic dispersion property of whistlers (higher phase speed at smaller spatial scales) is the key to understanding these results. The implications of these results for trying to model the global dynamics of the magnetosphere are discussed.

Nonlocal stability analysis of the MHD Kelvin‐Helmholtz instability in a compressible plasma

Abstract: A general stability analysis is given of the Kevin-Helmholtz instability, for the case of sheared MHD flow of finite thickness in a compressible plasma which allows for the arbitrary orientation of the magnetic field, velocity flow, and wave vector in the plane perpendicular to the velocity gradient. The stability problem is reduced to the solution of a single second-order differential equation including a gravitational term to represent the coupling between the Kelvin-Helmholtz mode and the interchange mode. Compressibility and a magnetic field component parallel to the flow are found to be stabilizing effects, with destabilization of only the fast magnetosonic mode in the transverse case, and the presence of both Alfven and slow magnetosonic components in the parallel case. Analysis results are used in a discussion of the stability of sheared plasma flow at the magnetopause boundary and in the solar wind.

Electron-Cyclotron Maser Driven by Charged-Particle Acceleration from Magnetic Field-aligned Electric Fields

Abstract: We present a detailed description of the auroral kilometric radiation (AKR) source region based on observations from the Fast Auroral SnapshoT (FAST) satellite and discuss how these new results may pertain to solar and stellar radio sources. FAST satellite observations are directly within the AKR source region and have unprecedented spatial and temporal resolution. They confirm many of the fundamental elements of the electron-cyclotron maser mechanism but with substantial modification. The most important modification is that the emissions do not draw their energy from a loss-cone instability; rather, the radiation results from an unstable "horseshoe" or "shell" distribution. The most far-reaching implication is that the electron-cyclotron maser is directly associated with a particular type of charged particle acceleration, a magnetic field-aligned (parallel) electric field in a dipole magnetic field. These findings change several of the characteristics of the electron-cyclotron maser mechanism and may necessitate reanalysis of some astrophysical radio sources. Under the shell instability, radio emissions with brightness temperatures ~1014 K, the steady state limit of the loss-cone instability, may be continuous. Through observations, we demonstrate that source brightness may be as high as 1020 K in steady state. A moderately or strongly relativistic beam may result in broadband emissions. A loss cone is not required, so the radiation source may be high above the stellar or planetary surface. Although the generation is in the X mode with k|| = 0, we suggest that the radiation, guided by a density cavity that is created by the parallel electric field, efficiently converts to the R mode, which experiences substantially lower absorption at higher harmonics. These findings also suggest that parallel electric fields may be a fundamental particle acceleration mechanism in astrophysical plasmas.

Collisionless magnetic reconnection in an asymmetric current sheet

Abstract: [1] The process of collisionless magnetic reconnection in an ion-scale current sheet containing strong gradients in the density and magnetic field strength across the layer is investigated using two-dimensional particle-in-cell simulations. Such a current sheet configuration contains a strong normal polarization electric field on the high field/low density (magnetospheric) side of the layer. In initial-value simulations for such an asymmetric sheet, the reconnection rate and saturation level are found to be smaller by factors of 2–3 compared with a similar-scale symmetric current sheet. These rates are probably too small to explain observations at the dayside magnetopause. The addition of an external-driving electric field increases the reconnection rate substantially. This driven reconnection configuration is characterized by a nearly parallel inflow of electrons along the magnetosheath separatrices as the electrons attempt to flow from the high density side to the low density side of the layer, a strong outward flow of Poynting flux along the magnetospheric separatrices associated with the normal electric field and out-of-plane magnetic field, and a strong ion outflow jet. The outflow region on the magnetospheric side also exhibits a patchy parallel electric field structure and parallel electron velocity distributions with a counterstreaming feature. The addition of a moderate uniform magnetic guide field component (shear angle ≳110°) has no appreciable effect on the reconnection rate but does produce a drift of the X line in the direction of the electron diamagnetic drift at a small fraction of the magnetosheath Alfven speed.

Ion-ion kink instability in the magnetotail: 2. Three-dimensional full particle and hybrid simulations and comparison with observations

Abstract: [1] The magnetotail current layer is thought to be subject to a variety of instabilities. One instability arising from the presence of two ion populations, the cold lobe ions and the current-carrying hot plasma sheet ions, is the ion-ion kink mode. Detailed linear properties of this mode in the magnetotail were investigated by Karimabadi et al. [2003], where it was shown that the mode differs from the standard Kelvin-Helmholtz instability. In this paper the nonlinear properties of the ion-ion kink mode are investigated using three-dimensional (3-D) full particle and hybrid (fluid electron, kinetic ions) simulations. It is shown that this mode is primarily driven by a velocity shear arising from the presence of multiple ion populations. The instability saturates as a result of broadening of the current layer and reduction of the velocity shear. The instability, however, differs in important aspects from the standard Kelvin-Helmholtz instability (KHI). Its linear mode properties exhibit dependencies on the kinetic details of the secondary ion population and its nonlinear evolution is found to be significantly different from previous MHD and Hall MHD treatments of the instability as well as from the KHI. In particular, the usual formation of vortices and coalescence that occur for the Kelvin-Helmholtz instability are absent for the ion-ion kink mode. Although the lobe ions can form vortices, such vortices are localized and do not affect current-carrying hot plasma sheet ions. Recent Cluster observations of modulated and bifurcated current sheets [Runov et al., 2003] are discussed within the context of the ion-ion kink mode. Hybrid simulations with open boundary conditions and using the parameters for this event demonstrate a very good agreement between the wavelength, period, and amplitude of the ion-ion kink mode and the observed wave-like disturbance. It is shown that the “bifurcated” current sheet can be explained in terms of a traveling kink displacement in which the current has a single continuous displacement into both hemispheres.

Magnetospheric Multiscale Overview and Science Objectives

Abstract: Magnetospheric Multiscale (MMS), a NASA four-spacecraft constellation mission launched on March 12, 2015, will investigate magnetic reconnection in the boundary regions of the Earth's magnetosphere, particularly along its dayside boundary with the solar wind and the neutral sheet in the magnetic tail. The most important goal of MMS is to conduct a definitive experiment to determine what causes magnetic field lines to reconnect in a collisionless plasma. The significance of the MMS results will extend far beyond the Earth's magnetosphere because reconnection is known to occur in interplanetary space and in the solar corona where it is responsible for solar flares and the disconnection events known as coronal mass ejections. Active research is also being conducted on reconnection in the laboratory and specifically in magnetic-confinement fusion devices in which it is a limiting factor in achieving and maintaining electron temperatures high enough to initiate fusion. Finally, reconnection is proposed as the cause of numerous phenomena throughout the universe such as comet-tail disconnection events, magnetar flares, supernova ejections, and dynamics of neutron-star accretion disks. The MMS mission design is focused on answering specific questions about reconnection at the Earth's magnetosphere. The prime focus of the mission is on determining the kinetic processes occurring in the electron diffusion region that are responsible for reconnection and that determine how it is initiated; but the mission will also place that physics into the context of the broad spectrum of physical processes associated with reconnection. Connections to other disciplines such as solar physics, astrophysics, and laboratory plasma physics are expected to be made through theory and modeling as informed by the MMS results.

Neutral line model of substorms: Past results and present view

Abstract: The near-Earth neutral line (NENL) model of magnetospheric substorms is reviewed. The observed phenomenology of substorms is discussed including the role of coupling with the solar wind and interplanetary magnetic field, the growth phase sequence, the expansion phase (and onset), and the recovery phase. New observations and modeling results are put into the context of the prior model framework. Significant issues and concerns about the shortcomings of the NENL model are addressed. Such issues as ionosphere-tail coupling, large-scale mapping, onset trigger- ing, and observational timing are discussed. It is concluded that the NENL model is evolving and being improved so as to include new observations and theoretical insights. More work is clearly required in order to incorporate fully the complete set of ionospheric, near-tail, midtail, and deep- tail features of substorms. Nonetheless, the NENL model still seems to provide the best avail- able framework for ordering the complex, global manifestations of substorms.

Bursty Bulk Flows in the Inner Central Plasma Sheet

Abstract: High-speed flows in the inner central plasma sheet (first reported by Baumjohann et al. (1990) are studied, together with the concurrent behavior of the plasma and magnetic field, by using AMPTE/IRM data from about 9 to 19 R(E) in the earth magnetotail. The conclusions drawn from the detailed analysis of a representative event are reinforced by a superposed epoch analysis applied on two years of data. The high-speed flows organize themselves in 10-min time scale flow enhancements called here bursty-bulk flow (BBF) events. Both temporal and spatial effects are responsible for their bursty nature. The flow velocity exhibits peaks of very large amplitude with a characteristic time scale of the order of a minute, which are usually associated with magnetic field dipolarizations and ion temeperature increases. The BBFs represent intervals of enhanced earthward convection and energy transport per unit area in the y-z GSM direction of the order of 5 x 10 exp 19 ergs/R(E-squared).

On Solving the Coronal Heating Problem

Abstract: The question of what heats the solar corona remains one of the most important problems in astrophysics. Finding a definitive solution involves a number of challenging steps, beginning with an identification of the energy source and ending with a prediction of observable quantities that can be compared directly with actual observations. Critical intermediate steps include realistic modeling of both the energy release process (the conversion of magnetic stress energy or wave energy into heat) and the response of the plasma to the heating. A variety of difficult issues must be addressed: highly disparate spatial scales, physical connections between the corona and lower atmosphere, complex microphysics, and variability and dynamics. Nearly all of the coronal heating mechanisms that have been proposed produce heating that is impulsive from the perspective of elemental magnetic flux strands. It is this perspective that must be adopted to understand how the plasma responds and radiates. In our opinion, the most promising explanation offered so far is Parker's idea of nanoflares occurring in magnetic fields that become tangled by turbulent convection. Exciting new developments include the identification of the “secondary instability” as the likely mechanism of energy release and the demonstration that impulsive heating in sub-resolution strands can explain certain observed properties of coronal loops that are otherwise very difficult to understand. Whatever the detailed mechanism of energy release, it is clear that some form of magnetic reconnection must be occurring at significant altitudes in the corona (above the magnetic carpet), so that the tangling does not increase indefinitely. This article outlines the key elements of a comprehensive strategy for solving the coronal heating problem and warns of obstacles that must be overcome along the way.

Bursty bulk flows in the inner central plasma sheet: An effective means of earthward transport in the magnetotail

Abstract: High speed flows in the Earth's Inner Central Plasma Sheet (ICPS) occur during enhanced flow intervals that have been termed Bursty Bulk Flow (BBF) events. The importance of different flow magnitude samples for Earthward transport in the ICPS are statistically evaluated and several representative BBF's and their relevance to Earthward transport are discussed. The selection of BBF's is automated in a database and they are shown to be responsible for most of the Earthward transport that occurs within the ICPS. The BBF related transport is compared to the transport measured within the entire plasma sheet during the 1985 AMPTE/IRM crossings of the magnetotail. The results show that BBF's last only a small fraction of the time in the plasma sheet but can account for several tens of percent of the Earthward particle and energy transfer and possibly all of the Earthward magnetic flux transfer in the plasma sheet.